Liquid-crystalline medium

Details Liquid-crystalline medium Liquid-crystalline medium

2002-07-30

2004-05-25

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299630, C252S299660, C252S299670

Reexamination Certificate

active

06740369

ABSTRACT:

The present invention relates to a liquid-crystalline medium based on a mixture of polar compounds of negative dielectric anisotropy, and to the use thereof for electro-optical purposes and to displays containing this medium, in particular displays with active-matrix addressing based on the ECB effect and especially on the vertically aligned (VA) effect.
Matrix liquid-crystal (MLC)displays of this type are known. Non-linear elements which can be used for individual switching of the individual pixels are, for example, active elements (i.e., transistors). The term “active matrix” is then used, where a distinction can be made between two types.
1. MOS (metal oxide semiconductor) or other diodes on a silicon wafer as substrate.
2. Thin-film transistors (TFTs) on a glass plate as substrate.
The use of single-crystal silicon as substrate material restricts the display size, since even modular assembly of various part-displays results in problems at the joints, which is why type 2 is preferred. A distinction is made between two technologies: TFTs comprising compound semiconductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon. TFT displays usually operate as TN cells with crossed polarisers in transmission and are back-lit.
The term MLC displays here covers any matrix display with integrated non-linear elements, i.e., besides the active matrix, also displays with passive elements, such as varistors or diodes (MIM=metal-insulator-metal).
In liquid-crystal displays of this type, the liquid crystals are used as dielectrics, whose optical properties change reversibly on application of an electric voltage. Electro-optical displays which use liquid crystals as media are known to the person skilled in the art. These liquid-crystal displays use various electro-optical effects.
The principle of electrically controlled birefringence, the ECB (“electrically controlled birefringence”) effect or DAP (“deformation of aligned phases”) effect, was described for the first time in 1971 (M. F. Schieckel and K. Fahrenschon, “Deformation of nematic liquid crystals with vertical orientation in electrical fields”, Appl. Phys. Lett. 19 (1971), 3912). This was followed by papers by J. F. Kahn (Appl. Phys. Left. 20 (1972), 1193) and G. Labrunie and J. Robert (J. Appl. Phys. 44 (1973), 4869).
The papers by J. Robert and F. Clerc (SID 80 Digest Techn. Papers (1980), 30), J. Duchene (Displays 7 (1986), 3) and H. Schad (SID 82 Digest Techn. Papers (1982), 244) have shown that liquid-crystalline phases must have high values for the ratio of the elastic constants K
33
/K
11
, high values for the optical anisotropy &Dgr;n, and values for the dielectric anisotropy &Dgr;&egr; of from about −0.5 to about −5 in order to be suitable for high-information display elements based on the ECB effect. Electro-optical display elements based on the ECB effect have a homeotropic edge alignment.
However, liquid-crystal displays of this type have some disadvantages compared with the known active-matrix TN displays, in particular a high viewing-angle dependence of the contrast ratio and of the grey shades.
A more recent variant of the ECB displays is the active matrix display based on the VAN (vertically aligned nematic) effect and the VAC (vertically aligned cholesteric) effect. VAN displays have been described, inter alia, in S. Yamauchi et al., SID Digest of Technical Papers, pp. 378 ff (1989), and VAC displays have been described in K. A. Crabdall et al., Appl.Phys.Lett. 65, 4 (1994).
The more recent VAN and VAC displays, like the ECB displays already disclosed earlier, contain a layer of a liquid-crystalline medium between two transparent electrodes, the liquid-crystal medium having a negative value for the dielectric constant anisotropy &Dgr;&egr;. The molecules of this liquid-crystal layer have a homeotropic or tilted homeotropic alignment (i.e. substantially perpendicular to the electrode surfaces) in the switched-off state. Owing to the negative &Dgr;&egr;, realignment of the liquid-crystal molecular parallel to the electrode surfaces takes place in the switched-on state.
In contrast to conventional ECB displays, in which the liquid-crystal molecules have, in the switched-on state, a parallel alignment with a preferential direction which is uniform over the entire liquid-crystal cell, in VAN and VAC displays this uniform parallel alignment is restricted only to small domains within the cell. Disclinations exist between these domains, also known as tilt domains.
As a consequence of this, VAN and VAC displays have greater viewing-angle independence of the contrast and of the grey shades compared with conventional ECB displays. In addition, displays of this type are simpler to produce since additional treatment of the electrode surface for uniform alignment of the molecules in the switched-on state, such as, for example, by rubbing, is no longer necessary.
In contrast to VAN displays, the liquid-crystal media in VAC displays additionally comprise one or more chiral compounds, such as, for example, chiral dopants, which, in the switched-on state, induce a helical twist of the liquid-molecules in the liquid-crystal layer by an angle of between 0 and 360°. The twist angle in the preferred case is about 90°.
Also known are liquid-crystal display elements using the IPS effect (in plane switching), in which both dielectrically positive and dielectrically negative liquid-crystal media can be used. Likewise, the dyes in guest/host displays can be employed either in dielectrically positive or dielectrically negative media, depending on the display mode used.
A further type of liquid-crystal display in which dielectrically negative media are used are the so-called “axially symmetric microdomain” (ASM for short) displays, which are preferably addressed by means of plasma arrays (PALCDs, from “plasma-addressed liquid-crystal displays”).
The above-mentioned display elements, in particular those which operate on the VA effect, generally have relatively short response times. However, there is an increasing demand, in particular in TV and video applications, for displays having even shorter response times. This can in principle be achieved either by reducing the rotational viscosities or by reducing the layer thickness d in the display elements. In order to keep the d·&Dgr;n value in the required range, liquid-crystal media having higher values of the optical anisotropy &Dgr;n are therefore necessary for display elements having smaller layer thicknesses d.
In addition, the chemical resistance to moisture, air and physical influences, such as heat, radiation in the infrared, visible and ultraviolet region, as well as direct and alternating electric fields is important. Furthermore, LC (liquid crystal) phases which can be used industrially are required to have a liquid-crystalline mesophase in a suitable temperature range, low viscosity and the highest possible value for the voltage holding ratio.
An object of the invention is to provide a liquid-crystalline medium based on a mixture of polar compounds of negative dielectric anisotropy which at least substantially meets the above-mentioned requirements, in particular has low rotational viscosities and/or comparatively high values of the optical anisotropy &Dgr;n.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
It has now been found that these objects can be achieved if media according to the invention are used in displays.
The invention thus relates to a liquid-crystalline medium based on a mixture of polar compounds of negative dielectric anisotropy, characterised in that it comprises one or more compounds of the general formula I
in which
R
11
is an alkyl group having from 1 to 12 carbon atoms or an alkenyl group having from 2 to 12 carbon atoms, and
R
12
is an alkenyl group having from 2 to 12 carbon atoms.
Compounds of the formula I in which R
11
is C
1-10
-alkyl and R
12
is 3-butenyl are described in JP 09208503 A as co

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